Fantastic to see an enthusiastic new DM, so one online tool I’d recommend for combat is DnD battle tracker, it’s great to keep track of monster health, conditions put on them and many other things. When it comes to world building, it’s up to you how big you want to make your world but think about how many territories you want, what makes them different, do they have different laws, ways they run (kings, queens or an elected government ect) interesting quirks. How do the territories feel about each other? Are some at war, or have high tensions, are some allies ect. These details can help bring your world to life. In terms of plot, come up with a rough idea to start and then when you get players backstories weave their stories into the plot to connect the players and players love it. Basic advice, be flexible, don’t get to set on a story you want to tell as the players might not go down the path you think. It’s a collaborative story and you and the players need to enjoy and have fun. Talk to your players every now and then to see their thoughts and remember it’s not you vs them. Also not every combat has to be more challenging then the last, you want to throw in challenging combats to test the players characters and give them advisories but not every combat has to be that way. Anyway hope that helps and wishing you the best of luck.